The ceremony of crossing the line is an initiation rite in the Royal Navy, U.S. Navy, and other navies which commemorates a sailor's first crossing of the equator. Originally the tradition was created as a test for seasoned sailors to ensure their new shipmates were capable of handling long rough times at sea. Sailors who have already crossed the equator are nicknamed (Trusty) Shellbacks, often referred to as Sons of Neptune ; those who have not are nicknamed (Slimy) Pollywogs. "King Neptune and his court" (usually including his first assistant Davy Jones and her Highness Amphitrite and often various dignitaries, who are all represented by the highest ranking seamen) officiate at the ceremony, during which the Pollywogs undergo a number of increasingly disgusting ordeals (wearing clothing inside out and backwards; crawling on hands and knees on nonskid-coated decks; being swatted with short lengths of firehose; being locked in stocks and pillories and pelted with mushy fruit; crawling through chutes and large tubs of rotting garbage; kissing the Royal Baby's belly coated with axle grease, hair chopping, etc), largely for the entertainment of the Shellbacks. Once the ceremony is complete, a Pollywog receives a certificate declaring his new status. Another common status is the Golden shellback, a person who has crossed the equator at the 180th meridian (international date line).

In the 19th century and earlier, the line-crossing ceremony was quite a brutal event, often involving beating "pollywogs" with boards and wet ropes and sometimes throwing the victims over the side of the ship, dragging the pollywog in the surf from the stern. In more than one instance, sailors were reported to have been killed while participating in a crossing the line ceremony.

As late as World War II, the line crossing ceremony was still rather rough and involved activities such as the "Devil's Tongue" which would be an electrified piece of metal poked into the sides of those deemed pollywogs. Beatings were often still common, usually with wet firehoses, and several World War II Navy deck logs speak of untold sailors visiting sickbay after crossing the line.

Efforts to curtail the line crossing ceremony did not begin until the 1980s, when several reports of blatant hazing began to circulate regarding the line crossing ceremony and at least one death was attributed to abuse while crossing the line.